1. Field of the Invention
This invention relates to an apparatus for measuring biopermeability.
2. Statement of the Prior Art
The technology of biopermeability measurement is extensively utilized for photoelectric plethysmographs (finger tip volume sphygmographs) and the like. The photoelectric plethysmograph utilizes the facts that blood in capillaries abundantly present in subcutaneous tissues of the finger tip or the like absorbs red light very well and that volumetric variations of blood vessels cause permeability variations.
The volumetric variations of blood vessels correspond to variations of blood flow for the following reasons. The absolute value of permeability is determined by the optical absorbence and physical quantity of matter located in the position of measurement. Therefore, the measured value of permeability reflects the volumes of tissues and blood of an organism occupying a measurement zone. This is why the volumetric variations of blood vessels correspond to variations of blood flow.
Further, the volume of a blood vessel is proportional to the sectional area thereof. Generally, the waveform of a photoelectric plethysmograph reflects variations of the sectional area of the vessel. Variations of the sectional area of the vessel result from expansion and contraction of vessel membrane due to variations in the pressure in the vessel. Therefore, the waveform of a photoelectric plethysmograph is determined by blood pressure variations in the vessel and by expansibility of the vessel membrane with respect to blood pressure. The blood pressure is collectively influenced by the force which forces blood out from the heart, volume of vessel, elasticity-characteristics of the vessel membrane, vein pressure and tension in the smooth muscles present in the vessel membrane. These four factors have a complex reflection on the blood pressure waveform. If these factors deviate from their normal values due to various disease causes, characteristic changes are produced in the waveform. The waveform change pattern is used for diagnosis through pattern recognition technique.
The force which forces blood out of the heart, as noted above, refers to the capacity of the heart as a source of energy to force out blood to the aorta, and the volume of the vessel refers to the inner volume of the aorta viewed from the heart.
Prior art biopermeability measurement apparatus does not measure absolute permeability, and therefore has a simple construction of a light source and a photo-sensor. The biopermeability to light is usually very low, specifically about 1% or below. Besides, variation of the vessel volume from average vessel volume due to blood pressure variation is also low. Therefore, in order to ensure sufficient signal-to-noise ratio in the measurement, the biopermeability to light and range thereof have to be measured with a sensor section urged against the surface of the organism under measurement that is, in a state in which the vessel membrane is adequately squeezed by external pressure to increase volume changes. The urging of the locality of measurement does not only increase the amplitude of volume change component in the vessel but also helps keep stable the coupling of optical system and organism to each other, thus contributing to the stability of the base line of the waveform of measurement. Therefore, the optical system may be of a simple construction including only a light source and a light-receiving element, and a high performance sensor is not necessary.
In the prior art biopermeability measurement apparatus, however, usually the base line of the recording is stabilized, and therefore the sensor output signal is AC amplitude by an amplifier having a time constant on the order of 1.6 sec. This means that only a change in light transmitted through the organism is recorded, and also it is difficult to obtain a stable setting of a 100% light incidence state, i.e., a state of 100% incidence of light flux from a light source on a light-receiving element, which state is used as reference in the measurement of biopermeability. Therefore, calibration of absolute permeability is impossible. For this reason, the output of the prior art apparatus is irrelevant to the absolute value of permeability even though it may represent permeability variation, and therefore it is designed on the basis of a gain (or amplification degree) selected at the manufacturer's convenience. It is thus impossible to compare amplitude data obtained by measurements with different apparatuses, and comparison of measurements is most often done with respect to waveform patterns. Although data obtained with the same kind of apparatus can be compared, even such comparison is meaningless unless the individual apparatuses are calibrated.
Further, about two-thirds of the blood pressure data is constituted by average value as DC component, and the variation data constitutes only about one-third of the overall data. The average blood pressure value as DC component contains various data. Nevertheless, where the photo-sensor mounting structure is of pressure application type, measurement is done in a state in which the vessel volume is slightly reduced under external pressure because of the external pressure being applied to the vessel membrane by the urging of the same when the photo-sensor is mounted. In addition, the measuring condition varies with the extent of urging, which in turn varies for every measurement. Therefore, it is extremely difficult to obtain measurement under fixed conditions. This means that it is inevitable to use only the change component of measurement. What is more, even the change component is greatly influenced by changes in the external pressure due to urging which results in changes in the waveform pattern. This is a grave defect in that it causes erroneous diagnosis.
The present invention intends to overcome the above drawbacks inherent in the prior art biopermeability measurement apparatus, and its object, accordingly, is to provide a biopermeability measurement apparatus which permits continuous and accurate measurement of the absolute value of permeability to light transmitted through an organism under measurement even if the transmitted light has very weak intensity.
Another object of the invention is to provide a biopermeability measurement apparatus which permits automatic measurement of the absolute value of biopermeability.